(a) Measurement of the astronomical color index of
a star,
utilizing the ultraviolet, blue and yellow visual images over two
pre-set wavelengths obtained by photoelectric filtering. Other
standardized filter wavebands are also used.
[A84] (b) A system of stellar magnitudes devised by Johnson and
Morgan at Yerkes which consists of measuring an object's apparent
magnitude through three color filters: the ultraviolet (U)
at 3600 Å; the blue (B) at 4200 Å; and the "visual"
(V) in the green-yellow spectral region at 5400 Å. It is
defined so that, for A0 stars, B - V = U - B = 0; it is
negative for hotter stars and positive for cooler stars. The
Stebbins-Whitford-Kron six-color system (U, V, B,
G, R, I) is defined
so that B + G + R = 0.
[H76]

(a) Part of the electromagnetic spectrum immediately
above visible light (but below Gamma-rays and X-rays); it therefore
comprises a range of radiation of shorter wavelength and higher
frequency than those of visible light. (UV) [A84] (b)A form of electromagnetic radiation, shorter
in wavelength than visible light. Ultraviolet wavelengths range
between about 1 nm and 400 nm. Ordinary glasses are not transparent
to these waves; quartz is a much more effective material for making
lenses and prisms for use with ultraviolet.
Like light, ultraviolet radiation is produced by electronic
transitions between the outer energy levels of atoms. The distinction
between the two types of radiation is in fact physiological rather
than physical. However, having a higher frequency, ultraviolet photons
carry more energy than those of light. [DC99]

Ultraviolet Astronomy

Astronomy carried out in the waveband 300nm to
about 10nm. At these wavelengths, the atmosphere is opaque to
radiation and hence these astronomies have to be conducted from above
the Earth's atmosphere. The wavelength band 300 to 120nm can be
successfully explored using telescopes which form a natural extension
of optical techniques. At shorter wavelengths, new approaches have to
be taken because the mirror materials become nonreflecting. In
addition at short ultraviolet wavelengths,
< 91.2 nm, the
inrerstellar medium is likely to become opaque because of
Lyman-continuum absorption. [D89]

Stars that are brighter than the
horizontal-branch stars and bluer
than the giant-branch stars. [H76]

Ultraviolet Catastrophe

In the late nineteenth century it was
realized that the short-wavelength region of black-body radiation
could not be explained by the theories of physics of the time
(classical physics). The problem - sometimes called the ultraviolet
catastrophe - was resolved by the concept of quantization of energy.
(see Planck's Radiation Law) [DC99]

Property of a star that emits more ultraviolet
radiation than one would have expected, based on its visual color. In
general, the greater the ultraviolet excess, the lower the star's
metallicity, because metals in a star's atmosphere absorb ultraviolet
radiation. [C95]

(a) The principle that the fundamental uncertainty
in a variable times that in its canonical conjugate is of the order
of Planck's constant: xp = h. Thus the uncertainty in the
measurement of the position of an electron varies inversely as
the uncertainty in the measurement of its momentum. A corollary is
that it is impossible to measure an atomic or nuclear
process without at the same time disturbing or altering the
process. [H76] (b) Principle of quantum mechanics, discovered by
Heisenberg, that there are features of the universe, like the position
and velocity of a particle, that cannot be known with complete
precision. Such uncertain aspects of the microscopic world become ever
more severe as the distance and time scales on which they are considered
become ever smaller. Particles and fields undulate and jump between all
possible values consistent with the quantum uncertainty. This implies
that the microscopic realm is a roiling frenzy, awash in a violent
sea of quantum fluctuations. [G99] (c) Also called the Heisenberg Uncertainty
Principle, a fundamental result of quantum mechanics stating that
the position and speed of a particle cannot be simultaneously known
with complete certainty. If one is known with high certainty, the
other becomes very uncertain. The product of uncertainty in position
and uncertainty in momentum (mass multiplied by speed) is equal to a
constant, Planck's constant. Since both initial position and initial
speed are required to forecast the future position of a particle, the
uncertainty principle prevents completely accurate predictions of the
future from the past, even if all the laws of physics are known. [LB90] (d) The uncertainty principle is a consequence of quantum
theory. It implies
that a pair of observables cannot both be measured simultaneously to
arbitrary accuracy. It can often be used to understand quantum theory
results in a simple way. [K2000]

Unification

Scientists have sought for centuries to unify the
descript ions of apparently
different phenomena by showing that they were due to the same underlying
natural laws and that complex levels of matter were made of simpler
levels. This unification process is a subject of very active research
about the forces of nature today. The possible unification of the
strong, electromagnetic, and weak forces is called a grand
unification. There is a continuing effort to unify these forces with
gravity. String theories seem to do that successfully. [K2000]

Unified Field Theory

A single theory to account for the
electromagnetic, gravitational, strong, and weak interactions by one
set of equations. So far, attempts to find such a theory have been
unsuccessful, although there has been some progress in unifying the
weak and electromagnetic interactions. [DC99]

(a) In particle physics, any theory exposing
relationships between seemingly disparate classes of particles. More
generally, a theory that gathers a wide range of fundamentally
different phenomena under a single precept, as in Maxwell's discovery
that light and magnetism are aspects of a single, electromagnetic
force. [F88] (b) Any theory that describes all four
forces and all of matter within a single, all-encompassing
framework. Also called the Unified Field Theory. [G99] (c) Attempts to unite the theories of the strong,
electromagnetic, and weak forces of nature. Ultimately it is hoped
that gravity will also be incorporated in this scheme. [D89]

Uniform Vibration

The overall motion of a string in which it moves
without changes in shape. [G99]

The hypothesis that the extensive changes in the
earth, as evinced in the geological record, have resulted, not from
massive catastrophes, but from the slow operation of wind, weather,
volcanism, and the like over many millions of years. Compare
catastrophism. [F88]

(a) The local mean time of the prime meridian. It
is the same as Greenwich mean time, counted from 0 hours
beginning at Greenwich mean midnight. UT0 is uncorrected;
UT1 is corrected for the Chandler wobble; UT2 is corrected
both for the Chandler wobble and for seasonal changes in
Earth's rotation rate. [H76] (b) A measure of time that conforms, within a close
approximation, to the mean diurnal motion of the Sun and serves as the
basis of all civil timekeeping. UT is formally defined by a
mathematical formula as a function of sidereal time. Thus UT is
determined from observations of the diurnal motions of the stars. The
timescale determined directly from such observations is designated
UT0; it is slightly dependent on the place of observation. When UT0
is corrected for the shift in longitude (see Longitude; Terrestrial)
of the observing station caused by polar motion, the timescale UT1 is
obtained. Whenever the designation UT is used in this volume, UT1 is
implied. [S92]

Universality

The phenomenon whereby many microscopically quite
different physical systems exhibit critical point behavior in with
quantitatively identical features such as critical indices. [D89]

Universality Class

This is a way of classifying the behavior of
systems near the critical points of continuous phase transitions.
Systems in the same universality class have the same behavior in the
critical region, when an appropriate matching is made between the
physical variables, and have the same critical exponents. [D89]

The total celestial cosmos. According to Gott
et al. the
universe seems to be on a large scale isotropic, homogeneous,
matter-dominated, and with negligible pressure. The total proper
mass content of about 1023M (Sandage derives
1056 g from his determination of the deceleration parameter
q0) and radius of about 2 × 1028 cm are
the order of magnitude that most
cosmologists would accept if the universe is bounded. Total
mass contributed by luminous matter, about 3 × 1053 g (see
Mass Discrepancy). Age about 18 × 109 yr for a Hubble
constant
H0 = 55 km s-1 Mpc-1. [H76]

Unstable Equilibrium

Equilibrium such that if the system is
disturbed a little, there is a tendency for it to move farther from
its original position rather than to return. (see Equilibrium;
Stability) [DC99]

Upsilon Particle

(a) A very massive (9 GeV) meson built from a bottom
quark and
bottom antiquark. Discovered in 1977, it is a member of the most
massive family of particles known at present. [D89] (b) A type of heavy lepton. (see Leptons) [LB90]

Uranium

A toxic radioactive silvery element of the actinoid
series of metals. Its three naturally occurring radioisotopes,
238U (99.283% in abundance), 235U (0.711%), and
234U (0.005%), are found in numerous minerals including the
uranium oxides pitchblende, uraninite, and carnotite. The readily
fissionable 235U is a major or nuclear fuel and nuclear
explosive, while 238U is a source of fissionable
239Pu.
Symbol: U; m.p. 1132.5°C; b.p. 3745°C; r.d. 18.95
(20°C); p.n. 92; r.a.m. 238.0289. [DC99]

Uranium-Lead Dating

A method of radioactive dating used for
estimating the age of certain rocks. It is based on the decay of
238U to 206Pb (half-life 4.5 × 109
years) or 235U to 207Pb (half-life 0.7 ×
109 years). The technique is useful for time periods from
107 years ago back to the age of the Earth (about 4.6 ×
109 years). [DC99]

A series of nuclear reactions, primarily among the
iron group of elements, accompanied by a high rate of neutrino formation
and postulated as a cause of stellar collapse. Neutrinos
carry away energy quickly and invisibly, so this process was
named for the Urca Casino in Rio de Janeiro, which carried
away money the same way. [H76]

A large class of double-lined eclipsing binaries
with very short
periods (a few hours) whose spectra indicate
mass transfer. They are distinguished by the fact that their primary
and secondary minima are equal. They are all F or G
binaries on or near the main sequence. They may be the progenitors of
dwarf novae. [H76]

The component of a star's motion away from the
Galactic
center. If a star moves away from the Galactic center, the star's U
velocity is positive; if a star moves toward the Galactic center, the
U velocity is negative; and if the star moves neither toward nor away
from the Galactic center, the U velocity is zero. The Sun has a U
velocity of -9 kilometers per second, so the Sun is moving toward the
Galactic center at 9 kilometers per second. [C95]